A lithium-ion secondary battery including a positive electrode containing a lithium-iron-manganese-based composite oxide as a positive electrode active material and a negative electrode containing a material capable of intercalating/deintercalating lithium ions as a negative electrode active material is expected to be a secondary battery having a high energy density. For example, Patent Literature 1 discloses a lithium-ion secondary battery using a lithium-iron-manganese-based composite oxide for a positive electrode active material. On the other hand, Patent Literatures 2 to each disclose a technique relating to a coating on a surface.